An ion source is a device that causes gas molecules to be ionized and then accelerates and emits the ionized gas molecules and/or atoms in a beam toward a substrate. Such an ion beam may be used for various purposes, including but not limited to cleaning a substrate, activation, polishing, etching, and/or deposition of thin film coatings/layer(s). Example ion sources are disclosed, for example, in U.S. Pat. Nos. 6,359,388; 6,037,717; 6,002,208; and 5,656,819, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1–2 illustrate a conventional cold-cathode type ion source. In particular, FIG. 1 is a side cross-sectional view of an ion beam source with an ion beam emitting slit defined in the cathode, and FIG. 2 is a corresponding sectional plan view along section line II—II of FIG. 1. FIG. 3 is a sectional plan view similar to FIG. 2, for purposes of illustrating that the FIG. 1 ion beam source may have an oval and/or racetrack-shaped ion beam emitting slit as opposed to a circular ion beam emitting slit. Any other suitable shape may also be used.
Referring to FIGS. 1–3, the ion source includes a hollow housing made of a magnetoconductive material such as steel, which is used as a cathode 5. Cathode 5 includes cylindrical or oval side wall 7, a closed or partially closed bottom wall 9, and an approximately flat top wall 11 in which a circular or oval ion emitting slit and/or aperture 15 is defined. The bottom 9 and side wall(s) 7 of the cathode 5 are optional. Ion emitting slit/aperture 15 includes an inner periphery as well as an outer periphery. Deposit and/or maintenance gas supply aperture or hole(s) 21 is/are formed in bottom wall 9. Flat top wall 11 functions as an accelerating electrode. A magnetic system including a cylindrical permanent magnet 23 with poles N and S of opposite polarity is placed inside the housing between bottom wall 9 and top wall 11. The N-pole faces flat top wall 11, while the S-pole faces bottom wall 9. The purpose of the magnetic system with a closed magnetic circuit formed by the magnet 23 and cathode 5 is to induce a substantially transverse magnetic field (MF) in an area proximate ion emitting slit 15.
The ion source may be entirely or partially within conductive wall 50; and/or wall 50 may at least partially define the deposition chamber. In certain instances, wall 50 may entirely surround the source and substrate 45, while in other instances the wall 50 may only partially surround the ion source and/or substrate.
A circular or oval shaped conductive anode 25, electrically connected to the positive pole of electric power source 29, is arranged so as to at least partially surround magnet 23 and be approximately concentric therewith. Anode 25 may be fixed inside the housing by way of insulative ring 31 (e.g., of ceramic). Anode 25 defines a central opening therein in which magnet 23 is located. The negative pole of electric power source 29 is grounded and connected to cathode 5, so that the cathode is negative with respect to the anode. Generally speaking, the anode 25 is generally biased positive by several thousand volts. Meanwhile, the cathode (the term “cathode” as used herein includes the inner and/or outer portions thereof) is generally held at ground potential. This is the case during all aspects of source operation, including during a mode in which the source is being cleaned.
The conventional ion beam source of FIGS. 1–3 is intended for the formation of a unilaterally directed tubular ion beam, flowing in the direction toward substrate 45. Substrate 45 may or may not be biased in different instances. The ion beam emitted from the area of slit/aperture 15 is in the form of a circle in the FIG. 2 embodiment and in the form of an oval (e.g., race-track) in the FIG. 3 embodiment. The conventional ion beam source of FIGS. 1–3 operates as follows in a depositing mode when it is desired to ion beam deposit a layer(s) on substrate 45. A vacuum chamber in which the substrate 45 and slit/aperture 15 are located is evacuated, and a depositing gas (e.g., a hydrocarbon gas such as acetylene, or the like) is fed into the interior of the source via aperture(s) 21 or in any other suitable manner. A maintenance gas (e.g., argon) may also be fed into the source in certain instances, along with the depositing gas. Power supply 29 is activated and an electric field is generated between anode 25 and cathode 5, which accelerates electrons to high energy. Anode 25 is positively biased by several thousand volts, and cathode 5 is at ground potential as shown in FIG. 1. Electron collisions with the gas in or proximate aperture/slit 15 leads to ionization and a plasma is generated. “Plasma” herein means a cloud of gas including ions of a material to be accelerated toward substrate 45. The plasma expands and fills (or at least partially fills) a region including slit/aperture 15. An electric field is produced in slit 15, oriented in the direction substantially perpendicular to the transverse magnetic field, which causes the ions to propagate toward substrate 45. Electrons in the ion acceleration space in and/or proximate slit/aperture 15 are propelled by the known E×B drift in a closed loop path within the region of crossed electric and magnetic field lines proximate slit/aperture 15. These circulating electrons contribute to ionization of the gas (the term “gas” as used herein means at least one gas), so that the zone of ionizing collisions extends beyond the electrical gap between the anode and cathode and includes the region proximate slit/aperture 15 on one and/or both sides of the cathode 5. For purposes of example, consider the situation where a silane and/or acetylene (C2H2) depositing gas is/are utilized by the ion source of FIGS. 1–3 in a depositing mode. The silane and/or acetylene depositing gas passes through the gap between anode 25 and cathode 5.
Unfortunately, the cold-cathode closed-drift ion source of FIG. 1 is problematic in the following respects. Cathode 5, along with the negative terminal of the power supply 29 (e.g., DC power supply), and wall 50 are all commonly grounded. Meanwhile, the positive terminal of the power supply 29 is electrically connected to the anode 25. During operation, as a result of this circuit, ions of carbon or the like from the feedstock gas tend to hit and coat the cathode during ion beam operation. This coating is insulating and thus causes sparking to occur which results in inefficient ion source operation (e.g., current goes down).
In view of the above, it will be apparent that there exists a need in the art for a technique for improving the efficiency and stability of ion source operation.